The present invention relates generally to methods and apparatus for controlling the flow of air entering aircraft inlets. Conventional commercial jet aircraft are powered by axial flow turbine engines that receive free stream air through an inlet. The air is then compressed in a series of axial flow compressor stages. Fuel is added to the compressed air in a combuster and ignited, generating a stream of high-enthalpy exhaust products. The exhaust products pass through a turbine (which powers the compressor) and then exit the aircraft through an engine nozzle. The expelled exhaust products, typically in combination with compressed air that bypasses the combuster and turbine, impart thrust to the aircraft.
The efficiency of the aircraft engine is determined in part by the uniformity of the flow entering the compressor from the inlet. If the flow entering the compressor contains pockets of low speed air or is otherwise distorted, the compressor will not operate at its peak efficiency. If the distortion becomes too severe, the compressor can stall and can cause the engine to fail. Accordingly, it is desirable to provide air flow to the compressor with the highest possible uniformity.
One factor contributing to the non-uniformity of air entering the engine compressor is the presence of a boundary layer in the aircraft inlet. The boundary layer is a region of flow immediately adjacent to the surface of the inlet that has a substantial velocity gradient, and occurs because the air immediately adjacent to the inlet surface must have a zero velocity (otherwise an infinite shear force would be generated at the surface), while the air distant from the inlet surface has a high velocity.
One approach for reducing the effect of the boundary layer on aircraft engine inlets is to reduce the length of the wetted surface ahead of the engine. Accordingly, many commercial transport aircraft include engines that are mounted on pods spaced apart from the aircraft fuselage and wing surfaces to avoid ingesting the boundary layers that tend to build up on these surfaces. However, in some instances, it may be desirable to position the inlet adjacent to either the wing or the fuselage. In these instances, the boundary layer developing over the surface forward of the engine is typically removed, for example, by sucking the boundary layer flow away through a porous surface, by energizing the boundary layer flow with high speed air jets, or by ducting the flow away with a boundary layer diverter. Systems that remove the boundary layer through a porous surface or energize the boundary layer can be expensive, difficult to maintain, and/or difficult to control. Diverting the boundary layer through a boundary layer diverter can provide a more cost effective solution, but can also divert air flow that may be advantageous during certain stages of aircraft operation, such as static operation and take-off.
During static and take-off conditions, the engine is typically operated at a high thrust setting, but the forward speed of the aircraft is relatively low. Accordingly, the engine may be unable to obtain sufficient air from the stream tube directly ahead of the inlet and instead must draw additional air from around the sides of the inlet and around the inlet lip. If the inlet lip is too sharp, this flow can separate and can create distortion at the compressor. One approach to addressing this problem is to make the lip blunter. However, a drawback with this approach is that the blunt lip can increase aircraft drag at cruise speeds. Another approach to addressing this drawback is to provide auxiliary doors in the inlet that provide additional air during static and low speed operation. However, a drawback with the auxiliary doors is that they can increase the complexity of the inlet while providing functionality for only a small fraction of the time the inlet operates.
The present invention is directed toward methods and apparatuses for controlling aircraft inlet air flow. A propulsion system in accordance with one aspect of the invention includes an external flow surface having a forward portion, and an engine inlet positioned at least proximate to the external flow surface and aft of the forward portion. The engine inlet can have an aperture positioned at least proximate to the external flow surface, and the system can further include an engine inlet duct extending aft from the aperture to an engine location. An auxiliary flow duct can be positioned at least proximate to the external flow surface, with the auxiliary flow duct having a first opening and a second opening spaced apart from the first opening. The first opening can be positioned to receive flow from the external flow surface during at least a first portion of an operating schedule of the propulsion system. The auxiliary flow duct can be configured to direct air to the engine location during at least a second portion of the operating schedule of the propulsion system.
In another aspect of the invention, the auxiliary flow duct can include a third opening between the first and second openings, with the third opening providing fluid communication between the auxiliary flow duct and the engine inlet duct. A valve, which can include a plurality of louvers, can regulate the flow of air through the third opening. In another aspect of the invention, a valve can be positioned at least proximate to the first opening of the auxiliary flow duct to control a flow of air through the first opening. In yet a further aspect of the invention, the external flow surface can include one of a lower wing surface and an upper wing surface, with the first opening of the auxiliary flow duct positioned proximate to the one wing surface and the second opening of the auxiliary flow duct positioned proximate to the other wing surface.
The invention is also directed to a method for controlling aircraft air flow. In one aspect of the invention, the method can include directing a first flow of air into an aircraft inlet aperture positioned proximate to an external flow surface of the aircraft and aft of a forward portion of the external surface. The method can further include directing the first flow through an engine inlet duct to an aircraft engine, receiving a second flow of air, including boundary area layer developed over the external flow surface, through a first opening of an auxiliary flow duct, and exiting at least a portion of the second flow of air from the auxiliary flow duct through a second opening of the auxiliary flow duct. The method can further include directing a third flow of air into the auxiliary flow duct, then from the auxiliary flow duct to the aircraft engine. In another aspect of the invention, the auxiliary flow duct can be a first auxiliary flow duct, and the method can further include directing a fourth flow of air through a second auxiliary flow duct to the aircraft engine.
FIG. 1 is a top isometric view of an aircraft having a propulsion system and inlet in accordance with an embodiment of the invention.
FIG. 2 is a side elevation view of an embodiment of the aircraft shown in FIG. 1.
FIG. 3 is a partially schematic, cross-sectional side view of a propulsion system having an auxiliary flow duct operating as a boundary layer diverter in accordance with an embodiment of the invention.
FIG. 4 is a partially schematic, cross-sectional side view of the propulsion system shown in FIG. 3 with the auxiliary flow duct operating to provide auxiliary air to an engine in accordance with another embodiment of the invention.
FIG. 5 is a partially schematic, cross-sectional side view of a propulsion system having an auxiliary flow duct in accordance with another embodiment of the invention.
FIG. 6 is a partially schematic, cross-sectional side view of the propulsion system shown in FIG. 5 providing auxiliary flow in accordance with an embodiment of the invention.
FIG. 7 is a partially schematic, cross-sectional side view of a propulsion system having an auxiliary inlet with a valve in accordance with another embodiment of the invention.
FIG. 8 is a partially schematic, cross-sectional side view of a propulsion system having an auxiliary flow duct with an additional opening into an engine inlet duct in accordance with another embodiment of the invention.
FIG. 9 is a partially schematic, cross-sectional side elevation view of a propulsion system having two auxiliary flow ducts in accordance with another embodiment of the invention.